Some polymers are known to degrade by hydrolysis in the presence of water and thereby decompose to smaller chemical units. Some of these polymers are also biodegradable, such as polylactic acid and polyglycolic acid. Polymers such as polylactic acid and polyglycolic acid can be referred to generally as polydioxanediones because each is prepared by polymerization of a dioxanedione-based monomer. As used herein, except as specifically noted otherwise, dioxaneone refers to compounds having a dioxane ring with at least one carbonyl oxygen pendant from the dioxane ring. The remaining three carbon atoms in the dioxane ring may have various constituents pendant therefrom. Although the term dioxaneone, which is also sometimes written as dioxanone, is often used in a specific sense to refer to 2-keto-l,4-dioxane, dioxaneone is used herein in a general sense as discussed below, unless otherwise specifically indicated by the general formula: ##STR1## where R.sub.1, R.sub.2, R.sub.3 and R.sub.4 can be any of a variety of constituents and where Z can be one or more constituents covalently bonded to the associated carbon atom in the dioxane ring. When all of R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are hydrogen and Z is two hydrogen constituents, then the compound is 2-keto-1,4-dioxane.
Dioxaneones such as lactide and glycolide, in which Z is a carbonyl oxygen, may be more specifically referred to as dioxanediones since they each have two carbonyl oxygens pendant from the dioxane ring. Dioxanediones are cyclic diesters that may be represented by the general formula: ##STR2## Where R.sub.1, R.sub.2, R.sub.3 and R.sub.4 can be any of a variety of constituents. When R.sub.1, R.sub.2, R.sub.3, and R.sub.4 are all hydrogen, then the compound is glycolide which is also referred to as 1,4-dioxane-2,5-dione. Although the term dioxanedione is sometimes used to refer specifically to glycolide, the term as used herein is always employed in the general sense to indicate a class of compounds as indicated by the generic formula above, except as otherwise noted herein. When R.sub.1 and R.sub.3 are methyl and R.sub.2 and R.sub.4 are hydrogen the compound is lactide, which may be also referred to as 3,6-dimethyl-l,4-dioxane-2,5-dione. A polydioxaneone having one or more repeating units representative of a dioxanedione monomer may be more specifically referred to as a polydioxanedione. When a dioxaneone contains one or more asymmetrical carbon atoms, such as is the case with lactide, then that particular dioxaneone can exist as various optical isomers. For example, lactide can exist as two optically active isomers, D-lactide and L-lactide, or as the optically inactive isomer meso-lactide. D-lactide and L-lactide can also be present in equal quantities to form an optically inactive mixture known as racemic-lactide. Both mesolactide and racemic-lactide are often designated as simply D,L-lactide.
Higher molecular weight polymers can be produced by ring-opening polymerization of dioxanedione monomers. Dioxanediones used as monomers to produce higher molecular weight polymers have traditionally been made from low molecular weight poly-.alpha.-hydroxycarboxylic acids by a depolymerization reaction often referred to as "backbiting." The backbiting process is relatively expensive, contributing to the lack of feasibility in developing low-cost consumer products for mass-market applications using polydioxanedione polymers.
Due to the expense and difficulty in preparing hydrolytically degradable polymers such as polydioxanedione, their use has been largely confined to high value medical applications where bioabsorbable materials are required. Most reported medical applications involve internal use of the polymers, such as for sutures, prosthetic devices, and drug release matrices. Some polymers that have received considerable attention for medical applications include polylactic acid, polyglycolic acid, poly-.epsilon.-caprolactone and polydioxanone.
Some attempts have been made in the medical field to vary properties of bioabsorbable polymers based on the specific intended use. Properties that have received some attention include strength, flexibility, and rate of hydrolytic degradation. It is generally known that a copolymer usually exhibits different properties from homopolymers of either individual comonomer. Some attempts have been made to develop specific copolymers for specific medical applications.
Many references, however, identify several possible comonomers without any consideration for the possible effects that such comonomers might have on properties of the copolymer. For example, U.S. Pat. No. 2,703,316 by Schneider, issued Mar. 1, 1955, discusses lactide polymers and copolymers capable of being formed into a tough, orientable, self-supporting thin film with up to 50% of another polymerizable cyclic ester having a 6- to 8-membered ring. The patent specifically discloses polymerization of 5 parts lactide and 5 parts glycolide and also polymerization of 12 parts lactide and 2 parts tetramethylglycolide, but also provides an extensive list of other possible comonomers with no elaboration on polymer properties.
A few references have suggested the use of hydrolytically degradable polymers outside of the medical field. For example, U.S. Pat. No. 4,057,537 by Sinclair, issued Nov. 8, 1977, discusses copolymers of L-lactide and .epsilon.-caprolactone prepared from a mixture of comonomers containing from about 50 to about 97 weight percent L-lactide and the remainder .epsilon.-caprolactone. Strength and elasticity are shown to vary depending on the relative amounts of L-lactide and .epsilon.-caprolactone monomers. Depending upon the L-lactide/.epsilon.-caprolactone ratio, the polymers are disclosed to be useful for the manufacture of films, fibers, moldings, and laminates. However, no specific applications are discussed. Sinclair discloses that plasticizers may be added to the copolymer if desired, but provides no guidance concerning what compounds might be suitable. Lipinsky et al , 1986, pp. 26-32, "Is Lactic Acid a Commodity Chemical," Chemical Engineering Process, August, discloses the use of polylactic acid for packaging material without discussing modifications of material properties by inclusion of plasticizers or other components.
Although it has been noted that suitable compounds, such as plasticizers, may be added to modify the properties of some hydrolytically degradable polymers, such as in U.S. Pat. No. 4,057,537 just discussed, little guidance has been given as to what compounds might be effective. Identifying suitable compounds for use in externally modifying and identifying suitable comonomers for internally modifying the properties of biodegradable polymers has been a major problem confronted in developing biodegradable polymers for mass-marketed products. Relatively few references discuss modification of properties of hydrolytically degradable polymers with external compounds, such as polylactide homopolymers and copolymers. The medical industry has generally sought to tailor polymer compositions to specific medical applications by developing specific copolymers, rather than to add external compounds. Those references that do discuss compounds, such as plasticizers, however, offer little guidance in selecting suitable compounds to be used for mass-marketed, hydrolytically degradable polymer products.
Compounds which effectively modify properties of polymer products are not to be confused with compounds that are designed only to aid polymer processing and that are removed prior to or during manufacture of the final product. Compounds which are effective in modifying properties of polymer products should be completely miscible with the polymer, nonvolatile, and should not migrate to the surface of the polymer composition, as might be desirable with a processing aid.
Plasticizers or other similar compounds used in mass-marketed products made of hydrolytically degradable polymers will be deposited into the environment in large quantities upon degradation of the polymers. Therefore even low levels of toxicity are a concern due to the potentially huge quantity of potential waste.
Thus, a need exists for degradable polymer compositions that are suitable for use with products that can replace existing non-degradable products that are rapidly becoming difficult to dispose of due to limited landfill space and other environmental concerns.